The present invention relates in general to an apparatus for powering X-ray tubes. More particularly, the invention relates to a high-voltage power supply that is adapted to operate from a variety of AC power sources, which power sources have a variety of voltages, phases and impedances, and to provide precise control of the high-voltage output signal provided to an X-ray tube.
In a conventional X-ray tube, X-rays are produced by generating electrons by thermionic emission from a tungsten filament (cathode). The electrons are then accelerated to an anode (which may be rotating for wear averaging purposes) to generate the X-rays. The emission intensity of the tube is controlled by the filament current and by the difference in potential between the anode and cathode. Current X-ray tubes may operate at potentials of up to 200 kV. These high operating voltages make control of the X-ray tube emission level a difficult problem, typically requiring expensive components. Furthermore, at high tube currents the voltage can fall very quickly, making precise measurement of the voltage difficult. Still further, at high voltages stray capacitive coupling occurs which prevents accurate measurement of the tube voltage.
Since present X-ray tubes typically require about 200 kV to be operated, a high voltage power supply utilizing a step-up transformer is required to raise the available AC line voltage to this level. The typical AC line voltage available in hospitals and clinics varies from single-phase 220 volts AC to three phase 600 volts AC. An X-ray power supply able to operate from widely different line voltages, phases and impedances would be desirable in that line matching of the transformer to the specific AC line voltage characteristics would not be required.
Precise control of the voltage and phase of the power supplied to an X-ray tube is important to ensure proper imaging for diagnostic purposes and to avoid unnecessary exposure of the patient to X-ray radiation which does not produce a useable image. For example, during a conventional radiographic gastrointestinal analysis, the patient ingests a radio opaque liquid containing barium. When the patient ingests the liquid, the doctor turns on the X-ray generating tube at a low level and positions the patient between the X-ray tube and a fluoroscopic screen. The doctor analyzes the patient's gastrointestinal tract while the barium flows through it. When the doctor sees a part of the procedure he wants to record, he typically replaces the fluoroscopic screen with a photographic plate and increases the X-ray to a level intense enough to expose the plate.
Precise phase control is also important when an X-ray image is to be recorded by a television camera. TV cameras have well established sweep rates to which the X-ray exposure must be synchronized. If the exposure is not synchronized, the resulting picture from the TV camera has an interference pattern or jitters, which will make the picture very difficult or impossible to view. An exposure synchronized with the 60 Hz sweep rate of the TV camera will produce a coherent picture. It is also permissible to use X-ray exposures of less frequent multiples of the 60 Hz rate, for example, 30, 15 or 7.5 Hz.
Single phase and three phase power supplies each have certain advantages, depending upon the exposure rate desired. For example, three phase power supplies are commonly employed to provide continuous X-ray emissions because the voltage ripple in the rectified signal from a single-phase supply is too large, causing the X-ray tube to produce a lower value X-ray than required. Filtering capacitors to eliminate ripple are generally impractical at the high voltages employed and interfere with switching on-off times. In contrast, single phase supplies are generally used to provide a short pulsed emission.
It would thus be desirable to provide a high voltage power supply for both continuous and pulsed X-ray emissions which provides a precisely controlled output voltage and which accommodates variations in the input AC line voltage.